1. Field of the Invention
The present invention relates to a waterproof casing, and more particularly to a waterproof casing suitable for housing an underwater camera.
2. Description of the Prior Art
Various equipment used in the water or rain are protected by waterproof casings. For example, in the field of photography, underwater cameras are commercially available, wherein a lens-fitted photographic film unit (hereinafter simply referred to as film unit) with a photographic film preloaded therein is encased in a waterproof casing. As is well know, the film unit consists of a film housing equipped with a simple photograph-taking mechanism and an outer casing formed of cardboard covering the film housing.
Such an underwater camera can take photographs at a depth of up to 2 to 3 meters, or in other moist environment such as in the rain or snow. The waterproof casing is constituted of a transparent box-shaped casing body and a lid, both molded from plastic, and is completely sealed. The film unit can be operated from outside the waterproof casing, since the waterproof casing is provided with operating portions for engagement with corresponding operating portions of the film unit. After all the frames are exposed, the underwater taking camera is forwarded to a photo laboratory, as it is, for photofinishing. The developed photographic film and resultant photoprints are returned to the user, but the waterproof casing and the film unit are not returned to the user.
In such a waterproof casing containing a film unit, when the ambient temperature changes, its internal pressure changes because of expansion or contraction of the air inside thereof. The change of the internal pressure causes failure of the film unit, or deformation of the waterproof casing, the deformation resulting in mechanical disconnection between the film unit and the waterproof casing.
In order to solve such problems, a pressure-control valve can be used for controlling the internal pressure of the waterproof casing. However, such a valve is expensive and requires a space for attachment, and so is not suitable for an underwater taking camera having a film unit.
Waterproof casings have been suggested in Japanese Utility Model Publ. No. 4-356 and Japanese Utility Model Laid-open Publn. Nos. 3-45535, 3-86338 and 4-70638, in which the internal pressure of the waterproof casing is controlled while overcoming the above problems of cost, space, etc. Such a waterproof casing is formed with an air flow opening which is covered by an air-permeable waterproof sheet. Air can pass but water cannot pass through the air-permeable waterproof sheet. However, the air-permeable waterproof sheet is a thin film formed of polymer, and so is easily torn. Furthermore, the air-permeable waterproof sheet is always exposed to the outside of the waterproof casing, so if a very thin and strong wire or the like were inserted into an air flow opening, the very thin air-permeable waterproof sheet (which is only several microns thick) would be pierced and torn.